8 Drainage Options 8.1 Surface Water This section outlines the proposals for the management of surface water runoff from the development in accordance with National and Regional policy requirements, and best practice guidance. 8.1.1
Policy Requirements and Design Criteria
National Policy in respect of Development and Flood Risk (PPS25) and the SFRA for East Hampshire District Council requires that for development proposals on sites comprising one hectare or above in Flood Zone 1, developers must assess the potential of the site to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water runoff. Both policies also recommend that appropriate SuDS measures should be considered as well as the overall layout and form of the development to reduce the overall level of flood risk in the area. As part the consultation process for the completion of the WCS, the EA has set out design criteria in respect of the management of surface water on site. As well as being guided by the specified design criteria, the surface water drainage strategy has also been prepared in line with the Building Regulations 2000, Part H and the best practice guidance document CIRIA C697 ‘The SuDS Manual’ (2007).
PPS25 outlines guidance on how to allow for the impacts of climate change in drainage design. Table B2, Annex B or PPS25 Table B.2 provides a precautionary response to the uncertainty about climate change impacts on rainfall intensity and recommends that an additional 30% be added to peak rainfall intensity to account for the impacts of climate change. Therefore, in accordance with this guidance, the proposed attenuation provision will be sized to allow for an increase of up to 30% in rainfall intensity. The EA has specified that as part of any design proposals, a SuDS based drainage strategy must be incorporated, which works with natural topography where possible. As a minimum, the following design criteria need to be satisfied:
Surface water attenuation will have to contain the surface water runoff from the 1 in 100 year (with an additional 30% for climate change) critical duration rainfall event
Discharge from surface water attenuation will have to be restricted to the Greenfield runoff rates, and may need to include for the provision of a complex control to ensure there is no increase in discharge from the site for low, and higher magnitude rainfall events.
Any proposed infiltration areas/ measures will require insitu infiltration testing, and the base of any such measures will have to be at least 1m above the seasonal maximum
81 Peter Brett Associates LLP
Whitehill Bordon Eco-town Detailed Water Cycle Study: July 2011